1. Field of the Invention
The present invention relates to a liquid crystal display device and, more specifically, to a lateral-electric-field mode active matrix liquid crystal display device which can achieve a high numerical aperture ratio.
2. Description of the Related Art
In general, a liquid crystal display device (LCD) is characterized to be thin, light, and low power consumption. Particularly, an active matrix liquid crystal display device (AM-LCD) which drives individual pixels arranged in length and width directions in a matrix form by active elements is recognized as a high-definition flat panel display. Especially, a display device (TFT-LCD) using thin film transistors (TFTs) as the active elements for switching the individual pixels is being spread widely.
Many active matrix liquid crystal display devices use the electro-optical effect of twisted nematic (TN) liquid crystals, which displays images through displacing liquid crystal molecules by applying, to the liquid crystal sandwiched between two substrates, an electric field that is roughly vertical with respect to the substrate face. This is called a “vertical-electric-field mode”. In the meantime, a “lateral-electric-field mode” liquid crystal display device which displays images through displacing liquid crystal molecules within a plane being roughly in parallel to the substrate face by applying an electric field that is roughly in parallel to the substrate face has been also well known from before. Various modifications are applied also on the lateral-electric-field mode liquid crystal display device as in the case of the vertical-electric-field mode liquid crystal display device, and an example thereof will be shown below.
Japanese Unexamined Patent Publication 2002-323706 (Patent Document 1) discloses a structure in which a pixel electrode and a common electrode (both formed in a comb shape) for generating a lateral electric field for driving liquid crystal are placed by sandwiching an insulating layer at a position higher (i.e., a position close to a liquid crystal layer) than a bus line (data line) which supplies a signal to an active element for driving each pixel (see claim 1, First embodiment, FIG. 1 and FIG. 2). With this structure, the electric field from the bus line can be blocked through forming the common electrode by covering the bus line, so that poor display caused due to vertical crosstalk can be prevented. Further, it is considered that the numerical aperture can be improved by forming the common electrode with a transparent conductive material.
FIGS. 19A-19C show an example of the structure of the typical lateral-electric-field mode active matrix liquid crystal display device. FIG. 19A is a plan view, FIG. 19B is a cross-sectional view taken along a line A-A′ of FIG. 19A, and FIG. 19C is a cross-sectional taken along a line B-B′ of FIG. 19A. Further, FIGS. 20A, 20B, 20C, and 20D are plan views of main parts showing manufacturing steps of the liquid crystal display device. All of those illustrations show regarding only one pixel region.
With the liquid crystal display device, as shown in FIG. 19A and FIG. 20B, each pixel region is formed in a rectangular range surrounded by a plurality of gate bus lines 155 extending in the lateral (left-and-right) direction of the drawings and a plurality of drain bus lines 156 extending in the vertical (top-and-bottom) direction of the drawings, and a plurality of pixels are arranged in a matrix form as a whole. A plurality of common bus lines 153 are formed by extending in the lateral direction of the drawings as in the case of the gate bus lines 155. A thin film transistor (TFT) 145 is formed at each intersection point between the gate bus lines 155 and the drain bus lines 156 by corresponding to each pixel. A drain electrode 141, a source electrode 142, and a semiconductor film 143 of the thin film transistor 145 are formed in respective patterns (shapes) shown in FIG. 20B.
Each of pixel electrodes 171 and common electrodes 172 for generating a liquid crystal driving electric field has comb-shaped parts (thin-strip sections projected within each pixel region) that are engaged with each other. There are two comb-shaped parts in the pixel electrode 171, and a single comb-shaped part in the common electrode 172 in this case. As shown in FIG. 19B, the pixel electrode 171 is electrically connected to the source electrode 142 of the thin film transistor 145 via a contact hole 161 opened through an organic interlayer film 160 and a protective insulating film 159. The common electrode 172 is electrically connected to the common bus line 153 via a contact hole 162 opened through the organic interlayer film 160, the protective insulating film 159, and an interlayer insulating film 157. A part of the source electrode 142 of the thin film transistor 145 overlaps with the common bus line 153 via the interlayer insulating film 157, and the overlapping part forms a storage capacitor for the pixel region.
The cross-sectional structure of the liquid crystal display device is as shown in FIG. 19B and FIG. 19C. An active matrix substrate and a counter substrate are joined and unified by having a liquid crystal 120 interposed there between.
The active matrix substrate includes a transparent glass substrate 111, and the common bus line 153, the gate bus line 155, the drain bus lines 156, the thin film transistor 145, the pixel electrode 171, and the common electrode 172 formed on the inner surface of the glass substrate 111. The common bus line 153 and the gate bus line 155 are formed directly on the inner surface of the glass substrate 111, and those are covered by the interlayer insulating film 157. The drain electrode 141, the source electrode 142, and the semiconductor film 143 of the thin film transistor 145 as well as the drain bus lines 156 are formed on the interlayer insulating film 157. Therefore, the common bus line 153 and the gate bus line 155 are electrically insulated from the drain electrode 141, the source electrode 142, the semiconductor film 143, and the drain bus lines 156 by the interlayer insulating film 157. Those structures formed on the glass substrate 111 are covered by the protective insulating film 159 except the contact holes 161 and 162. The steps generated because of the contact holes 161 and 162 are flattened by the organic interlayer film 160 formed on the protective insulating film 159. The pixel electrodes 171 and the common electrodes 172 are formed on the organic interlayer film 160. As described above, the pixel electrode 171 is electrically connected to the source electrode 142 via the contact hole 161, and the common electrode 172 is electrically connected to the common bus line 153 via the contact hole 162. The cross-sectional views of FIG. 19B and FIG. 19C are model illustrations, and those are not faithful regenerations of the actual step structure. The surface (the face on which the pixel electrodes 171 and the common electrodes 172 are formed) of the active matrix substrate having the above-described structures is covered by an alignment film 131 that is formed with an organic polymer film. Alignment processing for directing the initial direction of liquid crystal molecules 121 towards a prescribed direction (see an arrow in FIG. 19A) is applied on the surface of the alignment film 131. In the meantime, the counter substrate (a color filter substrate) includes a transparent glass substrate 112, a color filter (not shown) configured with three primary colors of red (R), green (G), and blue (B) formed on the inner surface of the glass substrate 112 by corresponding to each pixel region, and a light-shielding black matrix (not shown) formed in the regions other than those corresponding to each of the pixel regions. The color filter and the black matrix are covered by an acryl-based overcoat film (not shown). A columnar spacer (not shown) for controlling the space between the active matrix substrate and the counter substrate is formed on the inner surface of the overcoat film. Further, the inner surface of the overcoat film is covered by an alignment film 132 that is formed with an organic polymer. Alignment processing for directing the initial direction of liquid crystal molecules 121 towards a prescribed direction (see an arrow in FIG. 19A) is applied on the surface of the alignment film 132.
The active matrix substrate and the counter substrate having the structures described above are superimposed on one another with a prescribed space by being opposed to each other while having the surfaces where the alignment film 131 and the alignment film 132 are formed faced towards the inner side, respectively. The liquid crystal 120 is provided in the space between the both substrates, and the peripheral edges of the both substrates are sealed by a seal material (not shown) for sealing the liquid crystal 120 therein. A pair of polarization plates (not shown) is disposed on the outer side faces of the both substrates, respectively.
As described above, the alignment processing is uniformly applied on the surfaces of the alignment film 131 and the alignment film 132 so that the liquid crystal molecules 121 are aligned in parallel along the alignment direction when there is no electric field being applied. The alignment direction is defined as a direction that is tilted by 15 degrees clockwise with respect to the direction (the top-and-bottom direction of FIG. 19A) along which the comb-shaped parts of the pixel electrodes 171 and the common electrodes 172 are extended.
The directions of the transmission axes of the pair of polarization plates are set to be orthogonal to each other. One of the transmission axes of the pair of polarization plates is set to be consistent with the original alignment direction (the alignment direction when no electric field is applied) of the liquid crystal on which the alignment processing is applied uniformly.
Next, the manufacturing steps of the liquid crystal display device shown in FIG. 19 will be described by referring to FIG. 20.
The active matrix substrate is manufactured in a following manner. First, a chrome (Cr) film is formed on one surface of the glass substrate 111, and it is patterned to form the common bus line 153 and the gate bus line 155 having the shapes as shown in FIG. 20A. Thereafter, the interlayer insulating film 157 made of silicon nitride (SiNx) is formed all over the glass substrate 111 to cover the common bus line 153 and the gate bus line 155. Subsequently, the semiconductor film 143 (normally an amorphous silicon (a-Si) film) of the thin film transistor in an island pattern is formed in the interlayer insulating film 157 to overlap with the corresponding gate bus line 155 via the interlayer insulating film 157. Further, the drain bus line 156, the drain electrode 141, and the source electrode 142 are formed by forming a Cr film on the interlayer insulating film 157 and then patterning it (see FIG. 20B). Thereafter, the protective insulating film 159 made of SiNx and the organic interlayer film 160 made of a photosensitive acryl resin are formed in a stacked manner in this order on the interlayer insulating film 157 to cover the above-described structures. Subsequently, the square contact hole 161 opened through the protective insulating film 159 and the organic interlayer film 160 and the rectangular contact hole 162 opened through the interlayer insulating film 157, the protective insulating film 159, and the organic interlayer film 160 are formed (see FIG. 20C). Further, the pixel electrode 171 and the common electrode 172 are formed on the organic interlayer film 160 by forming an ITO (Indium Tin Oxide) film as a transparent electrode material on the organic interlayer film 60 and then patterning it. The pixel electrode 171 is in contact with the source electrode 142 via the contact hole 161. The common electrode 172 is in contact with the common bus line 153 via the contact hole 162 (see FIG. 20D and FIG. 19B). Thereby, the active matrix substrate can be fabricated.
The counter substrate (the color filter substrate) is manufactured in a following manner. First, the color filter and the light-shielding black matrix (both are not shown) are formed on one surface of the glass substrate 112. Thereafter, an overcoat film (not shown) is formed all over the glass substrate 112 to cover the color filter and the black matrix. Further, the columnar spacer (not shown) is formed on the overcoat film. Thereby, the counter substrate is fabricated. The alignment films 131 and 132 made of polyimide are formed, respectively, on the surfaces of the active matrix substrate and the counter substrate manufactured in the above-described manner. Thereafter, the alignment processing is done uniformly on the surfaces of the alignment films 131 and 132. Subsequently, after stacking the both substrates with each other with a prescribed space (4.5 μm, for example) therebetween, the peripheral edges of the both substrates are sealed by a seal material except the hole for injecting the liquid crystal. Then, after injecting prescribed nematic liquid crystal (nematic liquid crystal with refractive anisotropy of 0.067, for example) into the space between the both substrates from the hole for injecting the liquid crystal within a vacuum chamber, the hole for injecting the liquid crystal is closed. When the respective polarization plates (not shown) are laminated on the external surface of the both substrates after connecting and unifying the both substrates, the liquid crystal display device having the structures shown in FIG. 19 can be completed.
It is known that a region (called a “reverse rotation domain”) where the liquid crystal molecules rotate in the direction that is reversed from the normal rotating direction of the liquid crystal molecule at the time of applying a liquid crystal driving electric field is generated in the vicinity of the tip section of the comb-shaped part of the pixel electrode 171 and the common electrode 172 in the lateral-electric-field mode liquid crystal display device described above.
FIG. 21 is an illustration for schematically describing the principle of generating the reverse rotation domain in the liquid crystal display device shown in FIG. 19 to FIG. 20. For simplifying the explanation, FIG. 21 shows only the pixel electrodes 171, the common electrode 172, and the liquid crystal molecules 121, and liquid crystal electric fields (electric power lines) E generated within the pixel regions by the comb-shaped parts of the electrode 171 and 172 are illustrated schematically.
The direction of the rotation (this rotation occurs within a plane that is almost in parallel to the active matrix substrate and the counter substrate) of the liquid crystal molecules 121 generated by the liquid crystal driving electric fields E is defined by the relation between the initial alignment direction (the alignment processing direction shown by an arrow) of the liquid crystal molecules 121 and the direction of the liquid crystal driving electric fields E. Thus, it is in the “clockwise direction” in most of the areas in the pixel regions. B shows a border domain showing the border at which the liquid crystal driving electric fields E change.
However, the liquid crystal driving electric fields E become radial as shown in FIG. 21 in the vicinity of the tip sections of the comb-shaped parts of the pixel electrodes 171, so that the liquid crystal molecules rotate in the “counterclockwise” direction in the region (reverse rotation domains R) shown in the drawing. That is, the regions shown in the drawing are the regions (i.e., the reverse rotation domains R) where the liquid crystal molecules rotate in the “counterclockwise” direction.
Japanese Unexamined Patent Publication Hei 10-307295 (Patent Document 2) discloses a technique which reduces coloring in the display from an oblique viewing angle through bending electrodes that generate lateral electric fields to intentionally vary the liquid crystal driving (rotating) directions by each region with the bent parts when the electric field is applied (see claims 1, 3, 5 and FIGS. 1-2, 4, and 6).
For example, it is assumed to be in a structure in which the initial alignment direction of the liquid crystal molecules of a first sub-region and the initial alignment direction of the liquid crystal molecules in a second sub-region are the same, and the liquid crystal molecules in each sub-region rotate in the reversed rotating directions when a voltage is applied while keeping the alignment directions to be in a mutually symmetrical relation (see claim 3). Further, in this structure, it is preferable that the lateral electric fields for driving the liquid crystal molecules are generated by parallel electrode pairs and that the electrodes configuring the parallel electrode pairs are bent in a V-shape (see claim 5).
As described above, the liquid crystal driving electric fields E are distributed radially in the vicinity of the tip sections of the comb-shaped electrodes, thereby forming the regions (reverse rotation domains) where the liquid crystal molecules rotate in the direction opposite from the prescribed rotating direction due to the relation with respect to the initial alignment direction of the liquid crystal. Since the liquid crystal driving electric fields E are in a gradual radial form in the vicinity of the tip sections of the comb-shaped electrodes, dark regions (the border domains B) generated between the reverse rotation domains R and the normal domains become large and the positions of the border domains B are unstable as well.
Therefore, when there is an external pressure such as finger pressing or the like applied on the display surface, the shapes of the reverse rotation domains R (or the positions of the border domains B) become unstable. Therefore, it is recognized as a finger pressed mark even after the external pressure is released. Further, there is also an issue of generating panel transmittance loss since the width of the border domains B becomes widened. That is, while the reverse rotation domains R contribute to the panel transmittance, the border domains B remain in a dark state even at the time of white display (at the time of applying voltage).
It is therefore an exemplary object of the present invention to provide a structure which can achieve a high transmittance in a lateral-electric-field mode liquid crystal display device through stably controlling the domains generated in the terminal parts of the comb-shaped electrodes where the liquid crystal molecules rotate in the reverse direction.